1. Field of the Invention
The present invention relates to a microwave sensor. More particularly, the present invention relates to a solution for improving accuracy of detection of an object to be detected by the microwave sensor.
2. Description of the Related Art
Conventionally, as a security apparatus, a microwave sensor is known for sending microwaves to a detection area and, if a human body (an intruder) exists in the detection area, the microwave sensor receives waves reflected from the human body (microwaves modulated by the Doppler effect) to detect the human body see, e.g., Japanese Patent Application Laid-Open Publication No. 2003-121532).
Also, one type of microwave sensor is known for measuring a distance to an object utilizing a plurality of microwaves with different frequencies. This kind of sensor sends, for example, two microwaves with different frequencies to the detection area two microwaves with different frequencies to the detection area and detects a phase difference between two intermediate frequency (IF) signals based on respective reflected waves. The phase difference correlates with a distance to a detection target object, such as a human body, and tends to be increased as the distance to the detection target object becomes. Therefore, by finding the phase difference, the distance to the detection target object can be measured. Further, recognizing temporal change in the phase difference enables the determination of whether or not an object in the detection area is moving. In this way, for example, only an object moving in the detection area can be selected as the detection target object. Hereinafter, the detection operation for the phase difference between IF signals will be described for this kind of sensor.
If output signals based on reflected waves of two microwaves with different frequencies are sinusoidal waves IFout1 and IFout2 shown in FIG. 3(a) and FIG. 3(b) (which have a phase difference corresponding to a distance to a detection target object), rectangular waves W1 and W2 are formed from these IF output signals as shown in FIG. 4(a) and FIG. 4(b), respectively. By detecting a phase difference between the rectangular waves W1 and W2 (converted from a time difference Δt between rising edge portions of the rectangular waves in the figures), the distance to the detection target object can be measured. Further, recognizing a temporal change in the phase difference between the rectangular waves W1 and W2 enables the recognition of the movement of the object in the detection area.
If this kind of sensor is used as a security sensor and if an object moving in the detection area is only decided as the detection target object by recognizing the temporal change in the phase difference, the following problems are posed.
If this kind of sensor is installed outdoors, the phase difference between the rectangular waves W1 and W2 is generated by the swinging of plants or the like due to the wind and, thus, the plants or the like may be falsely detected (falsely alerted) as the detection target object. These situations are caused not only by the swinging of plants but also by rainfall or vibration of the sensor itself (vibration due to the wind). Similarly, if this kind of sensor is installed indoors, the phase difference between the rectangular waves W1 and W2 is generated by a rotating motion of an exhaust fan or the swinging of a blind or curtain and, also in this case, an object other than a human body may be falsely detected as the detection target object.
In consideration of these problems, the inventor of the present invention has already proposed a microwave sensor which can accurately distinguish the detection target object such as a human body from another non-detection target object for avoiding a false alert (see, e.g., Japanese Patent Application Laid-Open Publication No. 2003-207462). Specifically, a total move distance is recognized for the object in the detection area and, if the total move distance exceeds a predetermined distance, the object will be selected as the detection target object (since, in the case of plants, the total move distance does not exceed the predetermined distance). Hereinafter, such a countermeasure is referred to as “a plants countermeasure”.
As stated above, although it is effective to distinguish the detection target object from plants or the like by recognizing the total move distance of the object in detection area (the plants countermeasure), it is difficult to appropriately set a level for distinguishing the detection target object from plants (a plants countermeasure level: a distance as a decision value for distinguishing the detection target object from plants or the like).
In other words, if the plants countermeasure level has been set low (an object is detected even if the total move distance of the object is short), in the situation that plants are swinging in the range of a few dozen centimeters (cm), the plants may be recognized as the detection target object and a false alert may be generated. Especially, although this kind of microwave sensor is often used in combination with a passive infra-red (PIR) sensor which receives infrared radiation from a human body in the detection area and which detects an intruder from difference between the human body and an ambient temperature (as a combination sensor), if the plants countermeasure level is set low in this way, the microwave sensor may continuously generate false alerts. As a result, substantially no difference is made in comparison to the case that a sensor apparatus consists of the PIR sensor only, and credibility of the combination sensor may be reduced.
Contrary, if the plants countermeasure level has been set high (an object is not detected unless the total move distance of the object becomes long (e.g., on the order of 100 cm), it is difficult to detect a moving human body passing transversely across the detection area (a moving object with less change in a relative distance to the microwave sensor) and, also in this case, credibility of the microwave sensor can not be ensured.